The rims of rotationally molded containers in the past have generally been formed in a rolled over or flared configuration. Such a configuration may not be sufficiently rigid to be sturdy, and may not have an appropriate shape or surface area to attach accessories. It is therefore desirable to provide an enclosed polygonal rim for containers, such as refuse containers.
Rotational molding is a well known technique for molding hollow plastic containers. Rotational molding provides advantages over other molding processes, such as blow-molding and injection molding. For example, rotational molding produces strong, one-piece, low stress article that are not weakened by joints or seams, it allows relatively complex articles to be constructed, and it permits inserts and accessories to be more easily integrally molded into an article.
Rotational molding is accomplished melting particles of a thermoplastic resin, such as polyethylene or polypropylene in a heated, biaxially-rotating mold. The shape of the article to be molded generally corresponds to the interior shape of the mold. The thermoplastic resin particles melt and puddle in the bottom of the mold. As the mold is rotated simultaneously about two axes, all interior surfaces of the mold rotate through the melted plastic and unmelted particles, causing the melted plastic to coat the interior surfaces of the mold and conform to the mold in layers. The process continues with the multiple layers becoming progressively thicker until all of the melted plastic coats the interior of the mold.
After the plastic has melted and conformed to the mold's interior, the mold is moved to a cooling chamber where it is cooled by air and/or water. After the plastic has fully solidified, the mold is opened and the molded article is removed. Rough edges and unwanted sections of plastic are then trimmed off to give the article its final shape.
Rotational molding does have disadvantages, however. One major disadvantage of rotational molding is that it does not easily lend itself to the formation of additional or secondary enclosed voids. An inherent characteristic of rotational molding is that all surfaces of the mold must remain exposed to heat and to the molten plastic long enough to become coated with molten plastic. A section that has limited access for the molten plastic or heat and that is prematurely sealed off will not receive enough molten plastic.
Several techniques have been used to form secondary enclosed voids in rotationally molded articles. The most obvious method is to mold various components separately and then fasten them together to enclose a hollow void. However, this results in seams and joints, which sacrifice strength. Another method involves molding an article as one piece and then bending a portion of the article over itself to enclose a void. This results in at least one seam, however, in addition to possibly weakening the plastic where it is bent.
The most desirable technique thus far developed to form a secondary enclosed void in a rotationally molded article involves providing a relatively narrow gap or slot between the main interior portion of the mold and a secondary interior portion, in which the enclosed void will form. As the rotational molding process begins, the gap is open to allow passage of molten plastic and unmelted particles from the mold's main portion into the secondary portion. Therefore, the interior wall in the secondary portion will be coated with layers of molten plastic at the same time that the interior of the main portion of the mold is coated. Eventually, however, the thickness of the plastic layers increases to the point where the gap separating the two portions of the mold is completely filled or bridged with plastic. A wall of plastic therefore forms in the gap and separates the resulting enclosed void from the main interior of the molded article.
Thus the size and shape of the gap becomes very important. The larger the gap, the more molten plastic will flow into the enclosed void before it seals off. On the other hand if the gap is too large, the gap will not seal, leaving openings, and the object will be improperly formed.
During rotational molding to form an article having a secondary enclosed void, some of the air in the secondary portion of the mold is displaced by the inflowing plastic. At the beginning of the process, when the gap is still relatively open, the air in the secondary portion is simply displaced back through the gap into the main portion. However, as the layers of plastic build up to close the gap and form the wall that seals off the enclosed void, the enclosed void must be vented to allow the air to escape. Left unvented, air pressure (back pressure) in the enclosed void can prevent the walls of the rim from thickening sufficiently as the gap is closed off.
One approach to venting an enclosed void in a rotationally molded article is demonstrated in a prior art bin molded of high density polyethylene, which includes an integrally molded rim enclosing a hollow void throughout its circumference. The rim of this prior art bin is generally elliptical or teardrop-shaped, having rounded side walls. The mold for creating this prior art bin includes a circumferential gap or slot that extends continuously adjacent the top of the mold between the main portion and the rim portion of the mold. The rim portion of the mold has radiused interior walls that curve away from the gap. Such a curved wall shape may suffer drawbacks in strength and rigidity. Further, the shape is not conducive to the attachment of lids, handles, etc.
Further in the prior art bin, to vent the void within the rim during rotational molding, the gap in the bin mold between the main portion and the rim portion includes two enlarged, widened sections. These wide sections remain open during later stages of the molding process while the rest of the gap's circumference is being filled in to form the wall that encloses the rim void. The enclosed void of the rim is therefore vented into the interior of the main portion during molding.
One disadvantage of this venting method is that the wide sections of the gap, which serve as the vents, may not ever completely close during molding. If this occurs, the enclosed void in the rim will be open to the interior of the bin in the places where the separating wall did not completely form. This can cause several problems with the bin. For example, the strength, integrity, and rigidity of the bin will be compromised by the open gap in the bin's wall. Also, material within the bin will be able to leak into the interior of the rim through the gap, causing cleaning and sanitation problems.
Another disadvantage of the venting method used with this prior art bin involves the opposite situation. Instead of leaving an unfilled gap, the wide sections of the gap that acts as interior vents may close too early in the molding process. In this case, the purpose of providing vents will be negated. The result of premature vent closing will be that the rim walls will be formed substantially thinner than the walls of the main portion of the bin, thereby weakening the rim.
In one attempt to overcome this disadvantage (FIG. 6), a prior art bin's rim is formed having generally radiused or curved walls leading from the gap, so that molten plastic can easily flow into the rim during molding, ostensibly preventing premature gap closure.
As previously stated, the resulting curved shape of the rim has limitations from the standpoint of the rigidity and strength to the rim. Further, the curved rim surfaces do not provide a good lifting surface, nor are there any flat surface areas to which accessories and the like may be attached.
Therefore, there remains a need for new and improved rotationally molded container rim having a strong, rigid, enclosed, polygonal cross section. There also remains a need for containers having enclosed rims to be vented to the outside during molding in such a way as to ensure even and complete formation of both the rim itself and the wall separating the rim's hollow interior from the main portion of the bin.